"Economies of agglomeration" [0] have explained the phenomenon of urbanization for quite a while. It's not a stretch to think that there's an intellectual corollary. Why do university researchers work in centralized walkable enclaves?
Greater connectivity amplifies the productivity of headquarters cities by enabling them to more readily project their power and distribute their output globally. With easy access to the output of Silicon Valley/Seattle/New York, there's less reason for each Nowheresville to have its own knowledge-work economy. The once-powerful bank manager of your local Chase is replaced by an algorithm in New York. The once-lucrative IT systems engineering work at your local manufacturer is replaced by AWS in Seattle. And so on.
... which is a terrible, terrible thing and a recipe for the rise of disastrous politics or even the descent of the majority of humanity into a new dark age.
The phenomenon you describe is a problem. It's one of the greatest problems of the early 21st century.
Personally, I think it's a golden opportunity to transition from a nation of sprawled-out houses and freeways to one of dense condo towers and subways. The social and environmental damage of 50 years of car-centric white-flight suburban sprawl cannot be walked back soon enough; a mass migration to transit-centric megacities is one of few realistic prospects for averting climate change.
But the ascendant cities have decided not to accommodate any substantial population growth, so it's just another grotesque episode of gentrification and displacement, replacing city populations without really growing them, and only the most affluent from small cities and towns have a chance of making it in the more productive ones.
I'm not sure what the problem is. Land prices for multifamily-residential zoned parcels would reflect the most efficient multifamily-residential structures allowed. I'm not terribly concerned with the price of land in my neighborhood: I split ~5 houses' worth with 500 neighbors via a 35-story building.
Given the premise that cities have a fundamental advantage, what's the problem? Just wait a bit and liberals in cities will vastly outnumber rural conservatives, no?
This would eventually result in violent revolution. Suppressing that revolution will require a complete forfeit of civil liberties.
There's another reason it's bad too, and it's economic: in this world you will not be able to get ahead. These super-concentrated mega-cities will see runaway real estate inflation due to the "law of rent," while jobs will be scarce elsewhere. The choice will be: move to a "real" city and spend all of your surplus income on rent, or live elsewhere and have no opportunity to advance your career. The urban model looks a lot like the old "company store" model, which is a form of feudalism more or less.
In the end this future looks like a 1980s B-grade cyberpunk dystopia: endless slums and rural wastelands patrolled by drones and special forces officers and dotted by gated totalitarian enclaves for the super-rich. "DNA sample and body scan are required for entry..."
This is the direction we're headed if the cities that are absorbing the economy continue to reject population growth. But there's no reason they can't affordably include a much greater proportion of the population in denser structures.
I think Trump's rise isn't explicable only in the light of _economic_ stagnation in small towns. Growing insularity and racism (or maybe a climate where those are freely expressed?) are to blame as well.
Greater connectivity amplifies the productivity of headquarters cities by enabling them to more readily project their power and distribute their output globally. With easy access to the output of Silicon Valley/Seattle/New York, there's less reason for each Nowheresville to have its own knowledge-work economy. The once-powerful bank manager of your local Chase is replaced by an algorithm in New York. The once-lucrative IT systems engineering work at your local manufacturer is replaced by AWS in Seattle. And so on.
[0] https://en.wikipedia.org/wiki/Economies_of_agglomeration